Index scan uses incorrect bounds for MinKey and MaxKey

> db.test_min_max.find()

{ "_id" : "a_max_key", "a" : { "$maxKey" : 1 } }

{ "_id" : "a_min_key", "a" : { "$minKey" : 1 } }

{ "_id" : "a_null", "a" : null }

{ "_id" : "a_number", "a" : 4 }

{ "_id" : "a_subobject", "a" : { "b" : "hi" } }

{ "_id" : "a_undefined", "a" : undefined }

{ "_id" : "a_string", "a" : "hello" }

> db.test_min_max.find({a: {$gt: MinKey}})

{ "_id" : "a_min_key", "a" : { "$minKey" : 1 } } . // THIS IS WRONG

{ "_id" : "a_undefined", "a" : undefined }

{ "_id" : "a_null", "a" : null }

{ "_id" : "a_number", "a" : 4 }

{ "_id" : "a_string", "a" : "hello" }

{ "_id" : "a_subobject", "a" : { "b" : "hi" } }

{ "_id" : "a_max_key", "a" : { "$maxKey" : 1 } }

> db.test_min_max.find({a: {$gt: MinKey}}).explain()

{

	"queryPlanner" : {

		"plannerVersion" : 1,

		"namespace" : "test.test_min_max",

		"indexFilterSet" : false,

		"parsedQuery" : {

			"a" : {

				"$gt" : { "$minKey" : 1 }

			}

		},

		"winningPlan" : {

			"stage" : "FETCH",

			"inputStage" : {

				"stage" : "IXSCAN",

				"keyPattern" : {

					"a" : 4

				},

				"indexName" : "a_4",

				"isMultiKey" : false,

				"multiKeyPaths" : {

					"a" : [ ]

				},

				"isUnique" : false,

				"isSparse" : false,

				"isPartial" : false,

				"indexVersion" : 2,

				"direction" : "forward",

				"indexBounds" : {

					"a" : [

						"[MinKey, MaxKey]" . // THIS IS WRONG

					]

				}

			}

		},

		"rejectedPlans" : [ ]

	},

	"serverInfo" : {

		"host" : "franklinia",

		"port" : 27017,

		"version" : "0.0.0",

		"gitVersion" : "unknown"

	},

	"ok" : 1

}